In optical recording systems an optical beam is focused by an objective lens through a protective layer onto a recording layer of a disc. The objective lens is mounted in an electromechanical actuator which permits control electronics to move it independently along the focus and cross-track direction. Servo electronics utilize feedback signals, developed from the optical beam reflected from the disc, to determine how to accelerate the lens in order to maintain proper focus and tracking.
The quality of recording or retrieving data in many optical recording systems is generally very sensitive to the cross-track position of the recording spot on the medium. In a CD-R system, for example, data quality rapidly deteriorates when the recording or reading spot deviates from the centerline of the disc groove. A commonly-used technique for maintaining a focused spot on-track is referred to as push-pull tracking. The technique involves deriving a push-pull tracking error signal from an interference pattern caused by the interaction of the spot with the groove or other suitable tracking structure on the medium. A tracking servo adjusts the position of the spot to keep the push-pull signal at a predetermined optimum value generally referred to as a "tracking offset" or an on-track value. The tracking offset is typically determined by making a series of trial recordings during a calibration period before actual data is recorded. The tracking offset is intended to compensate for static errors such as detector and optical misalignment as well as electronic offset. However, additional tracking offset correction may be required due to media tilt in the cross-track direction. Media tilt changes from disc to disc and from point to point on a given disc.
Prior art such as the ones described by Horie et al. and Yoshimoto et al in U.S. Pat. Nos. 5,048,002 and 5,251,194, respectively disclose a method to null out the effect of tilt and other non-uniformity across the disc by moving the optical head to a plurality of different radial positions while the tracking servo is open. These radii shall be referred to as calibration radii.
FIG. 1 illustrates an open loop tracking error signal that is actually observed in an operational disc drive at a given calibration radius. The frequency modulation observed in FIG. 1 is caused by the eccentricity of the disc which causes the cross-track velocity to vary sinusoidally with respect to time. FIG. 1 also illustrates the tracking error signal upper envelope 6 and lower envelope 8 levels. From these envelope levels or the peak-to-peak of the open look tracking error signal, an optimum track offset is determined and subsequently applied to the tracking error so that the servo mechanism correctly positions the read or write spot on the track.
Although the calibration technique described in the above prior art compensates for certain disc non-uniformity, it has been observed that it does not fully compensate for cross-track tilt that may be present between the media and the optical head. In other words, the track error offset determined from the upper and lower envelopes of the tracking error signal is not always proportional to cross-track tilt. Therefore, the calibration technique of prior art may be unable to maintain the beam sufficiently on-track in the presence of disc tilt.
A need exists for a tracking offset technique in which a recording beam is maintained on-track as a function of disc tilt.